This invention relates generally to optical communication systems, and more particularly to systems for stabilizing wavelength or frequency separation of light sources to render them suitable for a wavelength multiplexing or frequency multiplexing systems in coherent optical communication or direct detection optical communication systems.
A conventional wavelength or frequency separation stabilization systems for light sources is discussed in IEEE, International Conference on Communications, Conference Record 37.4.1, pp. 1219-1223 (1988). This paper is directed to fiber type Fabry-Perot resonators, or in other words, a kind of etalon, as a means for stabilizing frequency separation. This etalon is used as an optical frequency discriminator for converting an optical frequency to an intensity. The system stabilizes the optical frequency separation by detecting the change of the intensity of the output beam of the etalon that occurs when the optical frequency changes, and by using this signal as a control signal.
In accordance with the technique described above, frequency stability of the light sources is limited by stability of the etalon with respect to the changes in temperature, mechanical vibration, air pressure, and the like. It is therefore difficult to maintain stability for extended periods.
A system is therefore desirable which can facilitate a frequency separation stabilization which is stable for extended periods of time.
Wavelength division multiplexing or frequency division multiplexing optical communication systems using the above-described technique render another problem in that part of a photo-coupler output for combining the beams from a plurality of light sources is discarded without using it. The optical output is not, therefore, utilized effectively.
It is also therefore desirable that a system be provided which can accomplish wavelength division multiplexing or frequency division multiplexing optical communication which utilizes effectively optical outputs of light sources.